Origin: 1250–1300; ME posterne < OF, var. of posterle < LL posterula, dim. of postera back door, n. use of fem. of posterus coming behind. See posterior, -ule
c.1290, "back door, private door," from O.Fr. posterne, earlier posterle, from L.L. posterula "small back door or gate," dim. of L. posterus "that is behind, coming after, subsequent," from post "after."